FIG. 3 is a block diagram of conventional composite duplexer 1. Composite duplexer 1 includes diplexer 4 connected with antenna 2, single-pole-double-throw (SPDT) switch 6 for low frequency bands connected with diplexer 4, and single-pole-double-throw (SPDT) switch 8 for high frequency bands connected with diplexer 4. SPDT switch 6 is connected with receiving terminal 10 and transmitting terminal 12 for low frequency bands, and switches the terminals. SPDT switch 8 is connected with receiving terminal 10 and transmitting terminal 12 for high frequency bands, and switches the terminals. A receiving circuit and a transmitting circuit are connected with receiving terminal 10 and transmitting terminal 12, respectively.
Composite duplexer 1 can be used in two transmission systems: the GSM900 system and the GSM1800 system using frequencies different from each other. Diplexer 4 separates a working frequency band in the GSM900 system from a working frequency band in the GSM1800 system.
Duplexer 1, when being used for the GSM900 system, a signal input from antenna 2 is separated by diplexer 4, is input to SPDT switch 6 for the low frequency bands, and output from receiving terminal 10 for the low frequency bands to be input to the receiving circuit. A signal output from the transmitting circuit is input to SPDT switch 6 from transmitting terminal 12 for the low frequency bands, and is output from antenna 2 via diplexer 4. SPDT switch 6 switches alternately receiving terminal 10 and transmitting terminal 12 for low frequency bands, thereby passing the input signal and the output signal to antenna terminal 4.
The frequency band in the GSM900 system is about 900 MHz, and the frequency band in the GSM1800 system is about 1.8 GHz. Thus, these frequency bands are widely separated, accordingly being separated by diplexer 4.
A GSM system, such as the GSM850 system in a frequency band of about 850 MHz, having low frequency band other than that of GSM1800 system, is very close to the frequency band of about 900 MHz in the GSM900 system, accordingly preventing diplexer from separating signals of the frequency bands.